This invention relates to a method and apparatus for recovering metals from metal plating baths with simultaneous neutralization of toxic effluents.
After metals have been plated onto substrates, the plated substrates are rinsed to remove a residual film of the electroplating bath. If this residual film is not removed, the plated substrates cannot be subjected to further treatment. This film is most often removed by rinsing the plated substrate in water. This rinse water is discharged as effluent, along with the used plating baths.
The effluents are first separated into streams containing cyanide and streams containing chromate. The cyanide stream is subjected to oxidizing conditions and the chromate stream is subjected to reducing conditions, after which both streams are subjected to precipitation of metal hydroxides contained therein. This sludge of metal hydroxides is generally considered to be useless.
Oxidation of the cyanides is usually effected with chlorine, sodium hypochlorite, ozone, or other ozidizing agents. Reduction of hexavalent chromium is most often effected with sodium bisulfite or sodium metabisulfite. Because of the strongly exothermic nature of these reactions and the possible evolution of toxic compounds, the concentration of cyanides in solutions to be treated should not exceed one gram per liter, and the concentration of hexavalent chromium in solutions to be treated should not exceed ten grams per liter.
Existing plating-effluents treatment plants utilize the aforesaid oxidizing and reducing agents, and the plants differ solely in the type of equipment used. Equipment in use includes concrete tanks sunk into the ground to receive by gravity the effluents from a metal finishing shop, free-standing reaction tanks into which effluents are mechanically pumped, or fully automated flow-through treatment plants using ion exchange or the Lancy process.
The Lancy process involves chemically rinsing the plated articles with dilute acid or alkaline solutions of the oxidizing or reducing agents used to treat the effluents, rather than rinsing the plated articles with water. In this process, the precipitation and separation of the sludge is effected within settling tanks located beyond the plating line.
With any of the methods described above, the sludge (mixture of metal hydroxides) precipitated during the final neutralization stage is concentrated by means of a filter press or a vacuum filter. This concentrated sludge is difficult to dispose of and cannot be reused.
Another important problem in metal finishing shops is treatment of the fumes exhausted by the ventilating system from the plating tanks. Treatment of these fumes is generally neglected for lack of appropriate methods and equipment.
Metals from concentrated plating solutions and effluents are mainly recovered by ion-exchange, electrodialysis, electrolysis, reverse osmosis, ultrafiltration, evaporation and the like. For concentrated solutions, these methods are impractical.
The aforesaid methods of effluent treatment in the metal finishing industry are used for the automatic and semi-automatic plating shops, as well as for manually operated plating shops. Existing methods of treatment of effluent from the plating shops, as well as recovery of metals therefrom, include no practical possibilities for treatment of toxic compounds exhausted by the ventilation system.